The natural hormone, 1α,25-dihydroxyvitamin D3 and its analog in ergosterol series, i.e. 1α,25-dihydroxyvitamin D2 are known to be highly potent regulators of calcium homeostasis in animals and humans, and more recently their activity in cellular differentiation has been established, Ostrem et al., Proc. Natl. Acad. Sci. USA, 84, 2610 (1987). Many structural analogs of these metabolites have been prepared and tested, including 1α-hydroxyvitamin D3, 1α-hydroxyvitamin D2, various side chain homologated vitamins and fluorinated analogs. Some of these compounds exhibit an interesting separation of activities in cell differentiation and calcium regulation. This difference in activity may be useful in the treatment of a variety of diseases as renal osteodystrophy, vitamin D-resistant rickets, osteoporosis, psoriasis, and certain malignancies.
In 1990, a new class of vitamin D analogs was discovered, i.e. the so called 19-nor-vitamin D compounds, characterized by the replacement of the ring A exocyclic methylene group (carbon 19), typical of the vitamin D system, by two hydrogen atoms. Biological testing of such 19-nor-analogs (e.g., 1α,25-dihydroxy-19-nor-vitamin D3) revealed a selective activity profile with high potency in inducing cellular differentiation, with very low calcium mobilizing activity. Thus, these compounds are potentially useful as therapeutic agents for the treatment of malignancies, or the treatment of various skin disorders. Two different methods of synthesis of such 19-nor-vitamin D analogs have been described (Perlman et al., Tetrahedron Letters 31, 1823 (1990); Perlman et al., Tetrahedron Letters 32, 7663 (1991), and DeLuca et al., U.S. Pat. No. 5,086,191). A few years later, analogs of 1α,25-dihydroxy-19-norvitamin D3 substituted at 2-position with hydroxy or alkoxy groups (DeLuca et al., U.S. Pat. No. 5,536,713) were synthesized. It has been established that they exhibit interesting and selective activity profiles. All these studies indicate that binding sites in vitamin D receptors can accommodate different substituents at C-2 in the synthesized vitamin D analogs.
In a continuing effort to explore the 19-nor class of pharmacologically important vitamin D compounds, analogs which are characterized by the transposition of the ring A exocyclic methylene group from carbon 10 (C-10) to carbon 2 (C-2), i.e. 2-methylene-19-nor-vitamin D compounds have been recently synthesized and tested (Sicinski et al., J. Med. Chem., 41, 4662 (1998); Sicinski et al., Steroids 67, 247 (2002); DeLuca et al., U.S. Pat. Nos. 5,843,928, 5,936,133 and 6,382,071). Molecular mechanics studies, performed on these analogs, showed that a change of ring-A conformation can be expected resulting in the “flattening” of the cyclohexanediol ring. From molecular mechanics calculations and NMR studies their A-ring conformational equilibrium was established to be ca. 6:4 in favor of the conformer that has an equatorial 1α-OH. Introduction of the 2-methylene group into 19-nor-vitamin D carbon skeleton changes the character of its (1α- and 3β-) A-ring hydroxyls; they are both now in the allylic positions, similar, to the 1α-hydroxyl group (crucial for biological activity) in the molecule of the natural hormone, 1 α,25-(OH)2D3. It was found that 1α,25-dihydroxy-2-methylene-19-norvitamin D analogs are characterized by significant biological potency, enhanced dramatically in compounds with an “unnatural” (20S)-configuration.
Recently, 2-ethylidene analogs of 1α,25-dihydroxy-19-norvitamin D3 have been synthesized. It turned out that such modification of the ring A resulted in significant biological potency for the compounds, especially enhanced in the E-geometrical isomers, Sicinski et al., J. Med. Chem., 45, 3366 (2002). Interestingly, it has been established that E-isomers have A-ring conformational equilibrium considerably shifted to one particular chair form, that possessing 1α-hydroxyl in an equatorial orientation.
Very recently, derivatives of 1α,25-dihydroxy-19-norvitamin D3 with 3′-hydroxypropylidene moiety at C-2 (DeLuca et. al, US Patent Application 20040229851) have been synthesized. Interestingly, their in vivo calcemic activity significantly exceeded that of 1α,25-(OH)2D3, especially in stimulating intestinal calcium transport. Molecular modeling studies of these analogs indicated that the presence of an oxygen function, located at the terminus of the propylidene fragment, could introduce additional interaction with the vitamin D receptor. In fact, the affinity of the synthesized compounds to VDR was increased and approached that of the natural hormone. Taking into account the recent findings on 2-methylene-1α-hydroxy-19-norvitamin D analogs with truncated side chain, Plum et al., PNAS, 101, 6900 (2004), indicating that these compounds effectively suppress parathyroid hormone levels, it was decided to further explore such modification of the vitamin D molecule.
As a continuation of the search for biologically active 2-alkylidene-19-norvitamin D compounds, analogs which are characterized by the presence of a 3′-hydroxypropylidene moiety at C-2 and a branched (1,1-dimethylpropyl) alkyl side chain containing no hydroxyl group have now been synthesized and tested.